1. Field of the Invention
The present invention is related to an LED driving circuit, and more particularly, to an LED driving circuit having a large operational voltage range.
2. Description of the Prior Art
Compared to incandescent lamps, light emitting diodes (LEDs) are characterized in low power consumption, long lifetime, small size and fast optical response. LEDs can easily be manufactured as miniaturized or array devices, which are widely used in various electronic products. Common LED applications include outdoor stationary displays (such as billboards, signboards or traffic signs) and portable devices (such as mobile phones, notebook computers or PDAs).
Reference is made to FIG. 1 for a voltage-current chart of an LED. When the forward-bias voltage of the LED is smaller than its threshold voltage Vb, the LED only conducts a negligible amount of current and the two ends of the LED are substantially open-circuited. When the forward-bias voltage of the LED is larger than its threshold voltage Vb, the current flowing through the LED exponentially increases with the forward-bias voltage and the two ends of the LED are substantially short-circuited. In an LED driving circuit, a current source is normally adopted for driving multiple LEDs so as to provide uniform luminescence.
Reference is made to FIG. 2 for a diagram of a prior art LED driving circuit 300. The LED driving circuit 300, including a voltage source VS and a current source IS, is configured to drive a luminescent device 10. The voltage source VS can provide a driving voltage Vf for turning on the luminescent device 10, while the current source IS can stabilize a driving current If which flows through the luminescent device 10 so as to maintain uniform luminescence. Since the LED is a current-driven device whose luminescence is proportional to its driving current, the luminescent device 10 normally includes a plurality of serially-coupled light-emitting diodes LED1-LEDn in order to provide sufficient and uniform light in large-size applications. Assuming all the light-emitting diodes LED1-LEDn have the ideal threshold voltage Vb, then a driving voltage Vf equal to n*Vb is required for turning on the luminescent device 10. In the prior art LED driving circuit 100, while more light-emitting diodes can provide higher light intensity, the forward-bias voltage of the luminescent device 10 also increases accordingly, thereby reducing the effective operational voltage range.
Reference is made to FIG. 3 for a diagram of another prior art LED driving circuit 400. The LED driving circuit 400, including a power supply circuit 110, a voltage detecting circuit 410 and a current-regulating circuit 420, is configured to drive a luminescent device 10. The power supply circuit 110 includes a voltage source VS and a bridge rectifier 20. The voltage source VS can output an alternating current (AC) voltage which periodically switches between positive and negative phases, while the bridge rectifier 20 is configured to convert the AC voltage outputted in the negative phase. The power supply circuit 110 can thus provide a direct current (DC) voltage Vf for driving the luminescent device 10, wherein the value of the driving voltage Vf periodically varies with time. The current-regulating circuit 420 includes a plurality of current sources IS1-ISn respectively configured to control the light intensity of corresponding light-emitting diodes LED1-LEDn in the luminescent device 10. The voltage detecting circuit 410 can detect the value of the driving voltage Vf, thereby turning on/off the current sources IS1-ISn of the current-regulating circuit 420 accordingly. Assuming all the light-emitting diodes LED1-LEDn have the ideal threshold voltage Vb: when the driving voltage Vf reaches the threshold voltage (Vb) of the light-emitting diode LED1, the voltage detecting circuit 410 turns on the current source IS1 and turns off the current sources IS2-ISn, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diode LED1 and the current sources IS1; when the driving voltage Vf reaches the overall threshold voltage of the light-emitting diodes LED1 and LED2 (2Vb), the voltage detecting circuit 410 turns on the current source IS2 and turns off the current sources IS1 and IS3-ISn, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diode LED1, the light-emitting diode LED2 and the current sources IS2; . . . ; similarly, when the driving voltage Vf reaches the overall threshold voltage of the light-emitting diodes LED1-LEDn (n*Vb), the voltage detecting circuit 410 turns on the current source ISn and turns off the current sources IS1-ISn−1, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diodes LED1-LEDn and the current sources ISn.
However, due to variations in material and manufacturing processes, the light-emitting diodes LED1-LEDn may not have the ideal threshold voltage Vb. The prior art voltage detecting circuit 410 is unable to control each current source according to the actual threshold voltage of a corresponding light-emitting diode. For example, assuming the actual threshold voltage Vb1 of the light-emitting diode LED1 is larger than the ideal threshold voltage Vb. If the voltage detecting circuit 410 turns on the current source IS1 when Vf=Vb, the light-emitting diode LED1 cannot be turned on. Thus for non-ideal light-emitting diodes, the voltage detecting circuit 410 is normally configured to turn on the current source IS1 when the detected driving voltage Vf reaches a switching voltage Vb′ larger than Vb. If the voltage detecting circuit 410 turns on the current source IS1 until Vf=Vb′, the extra voltage (Vb′−Vb1) not only increases the power consumption of the current source IS1, but also reduces the effective operational voltage range of the LED driving circuit 400.